cub: the young of a wild animal, still fed by its mother and not yet hunting, and in nearly every use the cub of a lion
On its own this word simply means "cub", the young of a wild animal. The lion that English Bibles almost always attach to it comes from the word standing next to it, not from the word itself: the phrase behind "a lion's whelp" is literally "a cub of a lion". Only once, in Lamentations 4:3, are the mothers something other than lions.
When a person or a nation is called a cub, the point is never that they are small and sweet. It is that they are dangerous and growing:
- Judah is a lion's cub who has come up from the prey
- Dan is a lion's cub that leaps out from Bashan
- the princes of Judah are the cubs a lioness raises into hunters
Hebrew names lions at several stages of life, and the cub is the first of them. Above it stands the young lion (כְּפִיר), old enough to hunt and roar on its own, and then the grown lion (אַרְיֵה) and the lioness (לָבִיא). Ezekiel 19 walks that ladder on purpose: the mother rears her cubs among lions, and one of them becomes a young lion who learns to catch prey and devours people.
So the word holds helplessness and menace together. A cub still has to be fed, which is why Nahum pictures a lion filling his den with torn meat for his cubs, and why Lamentations can shame a starving city by saying that even jackals nurse their young. A cub is also the future of an animal everyone fears, which is why a dying father reaches for it as a blessing on his son.